1. Field of Invention
The present invention relates to a magnetic recording medium based on an electromagnetic conversion such as magnetic tapes, magnetic disks and so on.
2. Prior Art
A magnetic recording medium may be manufactured by depositing under vacuum conditions a thin ferromagnetic metal layer consisting mainly of Fe, Ni, Co or an alloy thereof on the surface of a base substrate composed of a non-magnetic material such as a plastic film, e.g. polyester, polyimide or the like, or an aluminum plate, a glass plate or the like, by means of deposition, ion plating, sputtering or other conventional techniques. The magnetic recording medium of this kind is suitable for high density recording such as recording of video signals. However, as the film thickness of the thin ferromagnetic metal layer is as small as 0.01 to 0.5 .mu.m, corrosion is apt to occur on the surface of the magnetic recording medium in instances where it is exposed to highly moist atmospheres, particularly to atmospheres in which condensation is formed due to large variations in temperature. When corrosion has progressed to a certain extent, it has an effect on electromagnetic conversion performance and degrades working performance such as head touch, abrasion resistance and so on. Attempts have been made to control corrosion with an anti-corrosive agent; known anti-corrosive agents for a bulk metal such as Fe, Co, Ni or the like may be so employed.
Examples of such known anti-corrosive agents include a basic nitrogen compound such as an alkylamine or an aliphatic acid salt of an alkylamine; a phosphorus compound such as an alkyl phosphate, an alkyl thiophosphate, an alkyl hydrogen phosphoric acid ester or a phosphorus ester; a sulphonate such as petroleum sulphonate, mahogany acid sulphonate or dinonyl naphthalene sulphonate; a particular ester such as sorbitan monooleate or methyl ester of an organic acid; a particular carboxylic acid derivative such as a metallic soap of petroleum acid or of abietic acid or .alpha.-mercaptostearic acid; or a so-called volatile corrosion inhibitor such as dicyclohexylammonium nitrite, diisopropylammonium nitrite or cyclohexylamine carbamate. These bulk metal anti-corrosive agents are disclosed, for example, in "Manual of Metal Surface Technology", New Edition, Edited by Metal Surface Technology Association (1963; Publisher, Nikkan Kogyo Shimbunsha), pp. 1417 to 1439.
An oxygen-containing thin ferromagnetic metal layer formed by introducing a minute amount of oxygen gas into the vacuum system during the formation of the thin ferromagnetic metal layer is higher in coercive force than a layer containing no oxygen. Although the oxygen-containing thin ferromagnetic layer also provides an improved adhesive strength between the thin layer and the substrate and a greatly enhanced corrosion resistance, its corrosion resistance under severe atmospheres still is not satisfactory. Accordingly, such an anti-corrosive agent has been denounced as being inapplicable under severe conditions. It has now been found that anti-corrosive agents described hereinabove as being suitable for a thin ferromagnetic metal layer containing no oxygen, do not in fact impart a very good corrosion resistance to an oxygen-containing thin ferromagnetic metal layer, and in some cases they tend to accelerate corrosion.
The conventional bulk metal anti-corrosive agents are such that they act directly on the metal and stabilize the state of the metal surface. This phenomenon is described, for example, in "Rust preventing management" by Seiichi Fujii, 1978, Vol. 12, pp. 11 to 17. It is considered, however, that the surface of an oxygen-containing thin ferromagnetic metal layer is covered with an oxide layer of the ferromagnetic metal, and that when such a known anti-corrosive agent is used it penetrates through the oxide layer to act on the metal present therein, whereby the oxide layer cannot fulfil its purpose as a protective layer.
Therefore, it is now desired to use a new anti-corrosive agent for an oxygen-containing thin ferromagnetic metal layer, which is different from the conventional anti-corrosive agents.
A magnetic recording medium of the thin ferromagnetic metal layer type as described hereinabove provides an extremely high recording density compared with the conventional coated-type magnetic recording medium. This high-density recording also requires a spacing loss to be minimized and narrowing the gap of a magnetic head and at the same time flattening the surface of the magnetic recording medium. The spacing loss is required to be below 500 .ANG., and preferably below 300 .ANG.. Although head touch and travel performance can be improved to a considerable extent by controlling the surface condition of the magnetic recording medium, it is essentially impossible to significantly decrease the contact area between the head and the medium, and to prevent an increase in a coefficient of friction.
The anti-corrosive agents to be used in the present invention contain a compound having a particular polar group bindable to a ferromagnetic metal ion. These anti-corrosive agents, by being adsorbed onto the thin ferromagnetic metal layer prevent rust from forming, but do not produce a very good effect in improving head touch and travel performance. It has been found that they rather have the drawback that they clog the gap of the head when applied in excess.
Although application of various lubricants has been proposed in order to reduce a coefficient of friction between the head and the magnetic recording medium, such lubricants excluding special ones do not exhibit the effect of rust resistance and in some cases they rather become a rust accelerating agent.